Downhole operations and the handling of downhole well tools in completed wells has always presented a certain challenge, especially when working in wells having a natural pressure that exceeds atmospheric pressure, necessitating the containment of the well at all times. A further challenge has been the maintenance of well bores which pass through production zones that are not well suited to continuous production. For example, a production zone which yields both water and oil or gas or any combination thereof may require relatively frequent repositioning of a lower end of a production tubing in order to recover oil or gas efficiently. Production zones which produce crude oil high in waxy compounds or asphaltines, or laden with salts, which tend to plug casing perforations and therefore require frequent treatment to maintain an economic flow of hydrocarbon are further examples of such production zones.
To date, the maintenance of such wells has proven time-consuming and expensive. For example, in wells which produce both oil, water and gas and/or water and gas and have a mobile water/hydrocarbon interface, the production of hydrocarbon gradually decreases over time until only water or gas is produced from the well. Relocation of the bottom end of the production tubing string is then required to recommence oil production. The relocation of the tubing string has been a complex process which involved many time-consuming and expensive steps that are well known in the art. It is not difficult to appreciate that there is a need for a more efficient and less costly system for producing oil or gas from such wells. Such a system is described in applicant's copending patent application incorporated herein by reference. The apparatus described in that patent application eliminates many of the shortcomings of prior art procedures for selectively producing fluids from wells, performing barefoot completions of well bores in sensitive zones, and other downhole operations using production tubing and tools that require axial displacement within a limited range in a well bore. At the time of filing that patent application, it was considered that the apparatus described in U.S. Pat. No. 4,867,243 which issued on Sep. 19, 1989 to Garner et al. would be suitable for effecting the axial displacement of the downhole well tools. It has now been recognized that such prior art tools for inserting mandrels through wellheads is not necessarily adequate or optimal for performing the axial displacement of such downhole well tools.
There are several reasons why such prior art tools are not optimal tools for this purpose. First, they are designed for inserting wellhead isolation mandrels into wellheads and withdrawing them from the wellheads after the well is serviced. Since wellhead isolation mandrels are of inconsequential weight, they are stroked through a wellhead relatively easily. Moving a tubing string of 4,500' (1,500 meters), which is not uncommonly encountered in handling downhole well tools, may require a force in excess of 50 tons. The force required is due not only to the considerable weight to be lifted but also to the extra force required to unseat anchors and/or packers supporting the tubing string. Such forces may subject the wellhead to potentially damaging stresses. Second, wellhead isolation tools provide no mechanism for rotating a downhole tubing string since rotation is not required for the insertion or withdrawal of a wellhead isolation mandrel. When manipulating a downhole tubing string, however, rotational movement is often required in order to release or set components such as packers, anchors, hangers and the like. Considerable rotational force may be required to accomplish the release of such components and it is therefore desirable to provide a mechanism for selectively rotating the downhole string as required.